I agree with this point, and it what I intended. The key to the Pareto rule is that it should drive personal effort. For sites like Facebook, I think that you would just attract the content drivers (the 20%), and then the other 80% follow.

Think about it in terms of the Tipping Point. Go to the mavens/connectors first. They have all the knowledge know everyone. They'll do the buzz marketing for you and persuade others to join. Another term would be early adopter. I know that I have personal done this for Meebo.